The present invention is directed to an output stage for a transmitter. More particularly, the present invention is directed to a cross coupled output stage for a transmitter which has high impedance.
Transmitters which transmit digital signals onto a physical medium such as a cable have their outputs often terminated externally by an impedance which is supposed to match the characteristic impedance of the physical medium. In such cases their output stages should look like high impedance as seen from the outside so that the effective impedance is just determined by the external termination. This is useful in order to improve the return loss for the transmitter. The return loss is a measure of the reflection due to an impedance discontinuity created where the physical cable meets with a chip on which the transmitter is placed.
One such output stage which can be used is shown in FIG. 1. This is a pull-pull current mode driver configuration.
This device receives differential current inputs 20, 22 from a digital/analog converter (not shown) which in turn receives digital inputs, representing data to be transmitted, by the digital logic prior to it. Differential signals are used in order to cancel any extraneous noise from the signal, as is known. In the same manner, two outputs are also provided by the two halves of this differential cascode output stage at TXP and TXN. These two outputs are connected to a transformer 24 on opposite sides of a center tap in order to produce an external output at the line side 26 of the transformer. The center tap is hooked to an external power supply VCC which provides the differential current to both TXP and TXN side of the transmitter.
Each half of this circuit includes an arrangement of four n-type mos transistors. Two of these transistors 11 and 12, or 15 and 16 have their sources and drains serially connected between the input and ground and the other two 13 and 14, or 17 and 18 have their sources and drains connected between ground and the output. Gates of corresponding transistors in the two pairs are connected together. Current mode drivers such as these are highly popular since they present a high impedance when viewed from the outside.
However, there is a problem in that at high frequencies, parasitic device capacitances act as shunts and reduce the impedance.
First, the drain-to-bulk capacitance of devices 13 and 17 causes a shunt to be formed at the two outputs TXP and TXN resulting in lower impedance at high frequencies. Also, the gate-to-drain capacitance of devices 13 and 17 causes a negative feedback loop, lowering the output impedance. As seen in FIG. 1, when the drain of transistor 13, which is also connected to the node TXP, has a voltage which increases, the voltage of the gate of the same transistor also increases due to the gate-to-drain capacitance. Since transistor 11 is a source follower, the gate of transistor 12 also has a voltage which increases so that the voltage at the gate of transistor 14 also increases causing the node TXP to have a reduced voltage. This results in a negative feedback loop which lowers the output impedance of the output stage. Since the main purpose of the circuit is to have a high impedance, this unwanted negative feedback works against the function of the output stage. As a result, the impedance is reduced and the return loss gets worse. Though the second effect due to the gate to drain capacitance causing a negative feedback loop can be reduced in some alternative implementation, for example by driving the gates of device 13 and 17 by a low impedance source, the first effect due to the drain to bulk capacitance cannot be gotten rid of.